Mattyman
Model: 0055181
Install Date: 2009-11-12

This unit has been having intermittent surging problems, noted by owner during the weekly unloaded exercise.  Unit will run smoothly for a few seconds, then surge for a few seconds, then smooth and continues the cycle.  When it was first checked out, valve lash was checked and it was all in spec (they were recently adjusted but checked to be sure they were still fine.). The unit started up and ran smoothly with no issues.  After letting it run for about 5-10 minutes, it started to surge.  It was only about a 2-3hz surge but you could definitely hear it.  Unloaded frequency was around 62.5Hz and playing with the governor adjustment springs did not resolve the issue.  Suspecting a fuel supply issue, the pressure was checked and it was high.  A regulator was installed and the pressure was set to 7" water column.  Starting and running, it would drop to around 6"wc but when tested the surging was no longer present. 

After a few months, the owner contacted us again to say that it was surging again.  There was an updated air box for this particular unit and thinking that may be an issue, it was swapped out along with a new air filter and spark plug.  The surging persisted.  Verified that choke was operating properly.

I'm out of ideas for this one and looking to see if anyone else has any ideas on what else to check.
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Crow
i had a frozen air filter........causing surging.............i put on another filter, so far so good..................crazy right?
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78buckshot
Sometimes the governor is touchy, it takes some back and forth adjustment to get it balanced, are you 100% sure that the rubber intake manifold/bellows/snorkel is not leaking? You stated the fuel pressure was high and a regulator added, I don't like the sound of that, can you describe the fuel type and supply system?
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Peddler
I agree that if you haven't had a regulator since 2009 then you shouldn't need one now!  You need to get a pressure gauge on the fuel and post what the pressure is at the demand regulator, static and running.  Then what the pressure to see what it is doing when surging. 
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Mattyman
Fuel is natural gas, fed from a utility. We have seen higher pressures in the past few years than what we experienced in the past. Previously supply pressure was around 7"wc. Now when testing for new installs and existing we are typically anywhere from 8"-13". When testing at the demand regulator it was 7" static and about 6" while cranking and running. When it would surge, manometer would move ever so slightly that it was hard to decipher difference. Maybe 0.25" wc.

We can double check the connection between the airbox and engine to ensure there is no leakage. Also, this happened previously above freezing temperatures so don't suspect a frozen air filter.

Even though the governor was adjusted and tweaked, it is possible that there is an issue with spring tension. Will be replacing those as well to see if there is any change.
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